Antenna structure



June 27, 1961 c o ANTENNA STRUCTURE 2 Sheets-Sheet 1 Filed July 28, 1959INVENTOR. v James R M flouya/ BY .5: @MM GM/MK June 27, 1961 J. R. MDOUGAL 2,990,547

ANTENNA STRUCTURE Filed July 28, 1959 2 Sheets-Sheet 2 Q a, 26 30 9 7 Mk9/ =fl f t t in INVENTOR.

v 2- v 3= C 1, 11 $5 James A? M flue/ya 2y 8 M MW United States PatentThis invention relates to antennas for electromagnetic wave propagationand more particularly concerns a dual dipole flush-mounted antennastructure particularly suited forandUHF operation. The invention isherein illu'strat ivelydescribed by reference to thepresently preferredform; however, it will be recognized that certain modifications andchanges therein withrespect to details may be made without departingfrom'the underlying essentials.

While the invention is especially suited for aircraft applicationsrequiring. flush-mounted antennas, it has a wide variety ofother-applications as well. One of its "special characteristics is thecapacityto operate either with .dualpolarization or with singularpolarization and as a directive radiator in either mode. Another objectvisito provide such an antenna which may be flush-mounted, which willhave. a relatively wide -band width and which,

for its degree of gain or directionality, may be made I comparativelysmall and light in weight.

3 A further object is to provide a simple antenna of thesecharacteristics which is relatively inexpensive to manufacture andislent to mass production including production by printed circuittechniques if desired.

' Still another object is a flush-mounted antenna suitable 7 for highspeed aircraft skin installations which may be ,relatively shallow inits depth perpendicular to the plane of the skinand which may be madeimpervious to humidity, temperature, altitude (i.e. atmosphericpressure), sand, -dust, etc. In order to achieve the objective of a:shallow flush mounted construction, the novel antenna structure isreadily adapted to be dielectrically loaded in .an artificial mannerwhich adds inappreciable weight to the structure,.- this purpose beingachievable by artificial dielectric loadingusingmetal elements in arelatively .simple arrangement.

Still another object is ;a relatively versatileantenna which may beenergized at any of different feed points as a convenientmeans to-adjustinputimpedance thereof. By simple switching arrangements the versatilityof vthe antenna as an alternatively dually or, singularly polar- .izeddevice is realized with no structural modifications orja djustments ofthe antenna itself for either mode of operation.

One feature of the invention resides in the antenna Isjtructurecombining a conductive radiation surface havingan elongated slot thereinwith a zig-zag' conductive elementrnounted in the slot and presenting abroadside radiation face in substantially coplanar relation to themerit-may be energized to produce singular polarization of iface oftheconductive radiation surface, Either or both v .the conductiveradiation surface and the conductive eleminimum transverse width betweenits ends and'in- .creases in width exponentially toward both ends, theslot preferably having a similar configuration of somewhatIlargenoutline; Because of its shape, this antenna has {been referred toas a bow-tie dual dipole antenna. The

ing-from the point of minimum transverse width toward successivelyadjacent and interconnected transverse mem-. bers of the,zigzag elementshould be designed with I logarithmically increasing thickness andspacing progress ice As a further feature, the slot. is backed by areflective cavity less than a quarter wave length in depth, the samebeing minimized in depth preferably by the use of artificial dielectricloading achieved by metallic posts projecting from the zig-zag element,towards the base of the cavity but not electrical contact with suchbase.

These and other features, objects and advantages of the invention willbecome more fully evident from. the following description thereof basedon the accompanying drawings. 7

FIGURE 1 is a front perspective view of the antenna.

FIGURE Z is a front perspective view of the zig-zag element with metalposts for artificial dielectric loading. FIGURE 3 is a front perspectiveview of the cavitybacked slotted conductive radiation surface.

FIGURE: 4 is a transverse sectional view taken 0 line 4-'4 in FIGURE 1.

FIGURE 5 is a view similar to FIGURE 4, illustrating a modifiedconstruction.

FIGURE 6 is a front perspective view illustrating one mode of antennaenergization for dually polarized operation'.

FIGURE 7 is a front perspective view of the antenna with a differentmode of energization for dually polarized operation.

FIGURE 8 is a diagram illustrating the design theory applicable to thezig-zag element.

FIGURE 9 is a transverse sectional view indicating printed circuit orequivalent technique for forming the zigzag element and the conductiveradiation surface slot configuration.

Referring to the first embodiment shown in FIGURES 1 to 4, inclusive,the conductive radiation surface 10 is formed by means which may, in apractical case, com- 'prise a rectangular metal sheet or block or mayactually comprise the skin of an aircraft or other conductive support. Aslot 12 of elongated proportions is formed in the radiation surface 10and an elongated zig-zag ele- -rnent' 14 is mounted in the slot andpresents a broadside radiation face which is substantially in coplanarrelation to the surface 10. This zig-zag element 14 comprises aplurality of transversely. extending members 14a which transverse widthintermediate its ends (i.e. preferably midway) and increases in widthexponentially toward those ends as shown. Moreover, the thickness of andspacing between the successively adjacent transverse members 14a shouldbe minimum substantially where the transverse width of the element isminimum and should increase logarithmically toward opposite ends of thezigzag element. While the spacing between the peripheral outline of thezigzag element and the edges of the slot is not critical, this spacingshould be quite small at the ends in order to provide a high capacitiveloading on the zig-zag element. The slot preferably has the sameexponential taper as the element, although this is not strictlycompulsory.

For broadside radiation in one direction, the slot 12 should, in thepreferred construction, be backed by a conductive cavity 16 comprisingthe rear wall 16a and the peripheral side walls 16b which extend aroundthe slot edge. This cavity should have a depth less than a 0 quarterwave length, which in physical dimensions may be minimized by fillingthe cavity with a dielectric material 18 as shown in FIGURE 5 or,preferably (because of the lesser weight involved), by using anartificial dielectric loading technique involving metal rods or posts 20mounted on certain of the transverse members 14a at intervalsnalong thelongitudinal axis of the zig-zag ele'r'nent'14; These electricallyconductive members 20 project-from the backside of the zig-zag elementtoward th'ecavity surface 16a, but do not contact such surface. Ifde'sired these posts may be supported by stand-oh insulators ordielectric footings 22 secured to the cavity surface 16a and mayactually serve as a means to physically support the zig-zag elementcentered in the slot.

Preferably, the slot is sealed and the zig-zag element is supported byand embedded in a dielectric panel or sheet-24 which closes the slot.Such an arrangement presents minimum-weight and also minimizes thecavity depth so that the antenna structure may be llush mounted in='an-' aircraft skin, for example, and will .not project far into theinterior of the aircraft. The solid dielectric filling technique shownin FIGURE 5 adds weight but is otherwise satisfactory. In any case, thedielectric rnaterial used should be a low-loss dielectric.

The overall length of the zigzag element should be equal to or slightlyless than one-half wave length at the'ceriter frequency of the operatingband to which it is applied. Various modes of energization for such anantenna structure may be employed. In FIGURE 6 both theslot, or'morecorrectly the surrounding conductive radiation surface in which the slotis formed, and the zigzag element itself are separately energized inorder to form a dual dipole arrangement with dual polarization.Theslotted radiation surface is connected to be energized by a coaxialtransmission line 26, one conductor of which is connected to the surfaceat a point 26a and the other conductor of which is connected to the sameat a point 26b, such points being located at corresponding positionsalongthe length of the slot at opposite sides thereof. Nominally theselocations are at the longitudinal midpoint of the slot, but as a matterof practice their position may be shifted along the length of the slotin order to obtain different input impedances suitable for matching theenergy source used in different cases. Likewise,

the zigzag element in this example is energized from a coaxialtransmission line 28, one conductor of which is connected to the endtransverse member of the zig-zag element at point 28a and the otherconductor of which is -connected to the adjacent point 28b of the slotedge, nominally midway between the transverse side of the slot. However,as in the case of the slot itself, impedance match may be achieved byadjusting the position of these connecting points along the end of thestructure, i.e. perpendicular to its length. Suitable switching means.30 may be interposed in thetransmission lines 26 and 28, between theantenna structure and the energy source (not shown) in order to connecteither or both transmission lines to the respective antenna elements ata given time.

.In this way it is possible to produce a dually polarized radiationpattern or a singularly polarized radiation pattern withone direction ofpolarization or the other.

In the modification shown in FIGURE 7 the zig-zag element 14' is formedin two parts separated at the longitudinal midpoint (i.e. 14s). Thetransmission line 28' in this instance has one conductor connected tothe inner end of one-half of the Zig-zag element at point 28a and itsother conductor connected to the adjacent inner end of the other half ofthe zig-zag element at point 28'b. In this case the transmission line26' is connected to the slotted reflective surface in the same manner asin FIG- URE 6.

FIGURE 9 illustrates the application of printed circuit techniques toforming the antenna cavity and the zig-zag element. The zig-zag elementin this case is designated 140 and the slotted conductive surface 100.

, Turningnow to FIGURE 8, a portion of the zig-zag element 14 is" shown,particularly an end :portion, to

illustrate the theoretical design considerations for optimum performanceof the antenna element as a dipole device having the characteristicsdescribed. The thickness of the successive transversely extendingmembers 14a and the intervening slots 14b is designated 1. withappropriate subscripts to indicate serial relationship. These slotsrepresent dielectric surfaces which intervene between the conductivesurfaces represented by the members 14a. The distance from thelongitudinal center axis xx transversely to the outside extremities ofthe slots or gaps formed between successively adjacent elements and alsothe corresponding outer extremities of the interconnecting portions ofconductor which join'together successively adjacent elements on eitherside of an individual slot are designated y with appropriate subscriptsand superscripts to designate serial relationship, as shown in the view.The dimensions t t t t are related tothe ordinate by the relationship:

where n denotes the numerical order of the surfaces (i.e. conductive ordielectric) as counted from the origin of thecoordinate system. Theinterval t should decrease in logarithmic fashion progressing from theorigin (where the origin is at the end of the zigzag element) as maybedemonstrated by solving. the above equation for x as follows:

1 A-B 1 A-B x Log e( tn or x= n3Log e( in The constants A, B, a, thedielectric constant of the material used 'in the sheet 24, the number ofintervals t the type of feed selected and the size and shape of thereflecting cavity are all design parameters which in any particularinstance will affect the impedance, pattern, power handling capacity andother characteristics of the antenna. The complete zig-zag configurationis obtained by detemiim'ng the design parameters for onehalf of theelement, duplicating the curve thus obtained, for the other half, andconnecting the two halves together in the center as shown in FIGURES 1and 2 or with a 'gap between the two halves as indicated in FIGURE 7.

The y dimensions are defined by the exponential functions indicated inthe diagram in FIGURE 8, from which the bow-tie designation is obviouslyderived.

It has been found that the radiation pattern of such an antenna isessentially independent of frequency over as much as a fifty percentband width variation and that the impedance is such as to be inherentlycompensated over a thirty percent band width variation, depending uponthe standards used, when the backing cavity 16 is approximately0.08.wave length deep, measured at the center frequency of the hand. Ifthe reflecting cavity is omitted, the resulting radiation pattern is, ofcourse, different. It is found as a matter of interest, that the slot 12should be cut only slightly longer and wider than the zig-zag elementand, as previously mentioned, may be although is not necessarily of thesame exponential taper. This is another design parameter which issubjectto some variation, with consequent eflfect on the impedancecharacteristics and radiation characteristics of the device. Aspreviously stated, the gap between the slotand the-zig-zag elementshould be quite narrow at the ends in order to provide capacitiveloading to the zig-zag element suitable for proper operation thereof asa dipole.

It should be noted that present-day theories applicable to antennadevices, specifically the log periodic theory, do not hold in this casesince the dimensions of this antenna structure are such that theinterval t in the mid region especially, is so small as to appear almostnegligible when compared with wave lengths even at the highest operatingfrequency of the antenna. A more applicable analysis is obtained bycomparison with an array of infinitesimally thin; connectedV-shapedelements with an assumed sinusoidal current distribution andwith mode of operation herein disclosed ,S uch theory proves to ,be.true in practice, indicating an extended range of "usefulness for theantenna.

It should beiffnoted that the optimum operation for broadside radiation[from the zig-zag element requires that the currents be essentiallyinphase a'longits length. condition is met in practice bycapacitivelygloading the zig-zag element by use of relat" ly narrow gapsbetween itscnd' members and'the a 'd i 1 tli number of transversemembers 1411 is increased, to the point where it is no longer possibleto maintain the inphase relationship of the currents. It is found thatthe isotropic pattern directivity or gain of the zig-zag element isapproximately 8 decibels when the antenna has optimum design and acavity depth of approximately 0.08 wave length.

It should also be noted that the zig-zag element may be operatedindependently in space (i.e., without a surrounding slotted radiationsurface), particularly when such element is backed by a reflectivecavity and when it is connected for energization in shunt with such acavity. Under these conditions the zig-zag element has a very workableinput impedance.

It is recognized that slot antennas as such are well known in the art.

These and other aspects of the invention will be evident to thoseskilled in the art based on the foregoing disclosure of the preferredform and mode of operation of the invention.

I claim as my invention:

1. Electromagnetic antenna structure comprising two radiating elements,one a means presenting a conductive radiation surface having anelongated slot therein, and the other a zig-zag conductive elementmounted in said slot and presenting a broadside radiation face insubstantially coplanar relation to said surface, said zig-zag elementhaving a plurality of members extending transversely to the legnth ofsaid slot and successively interconnected at alternately opposite endsto define an outline configuration contained within the outline of saidslot and all portions of which. are marginally spaced inwardly from theedge of said slot on all sides, and energy transmission meanselectrically connected to at least one of said elements to energize saidantenna.

2. The anenna structure defined in claim 1, wherein the transverse widthof the zig-zag conductive element is minimum intermediate its ends andincreases exponentially to its respective ends.

3. The antenna structure defined in claim 2, wherein the thickness ofand spacing between the successively adjacent transverse members of thezig-zag element is minimum substantially Where the transverse width ofthe element is minimum, and increases logarithmically toward oppositeends of said element.

4. The antenna structure defined in claim 3, wherein the spacing betweenthe slot and the zig-za-g element at the respective ends thereof isrelatively small and is materially less than the spacing therebetween atthe sides, whereby the zigzag element has a high capacitive loadingproducing a broadside radiation pattern.

5. The antenna structure defined in claim 4, and means defining ashallow conductive cavity behind the slot and substantially enclosed onall sides extending around the slot and on the back side extendinggenerally parallel to the slot, said cavity having less than aquarter-wave length depth, transverse to the general plane of the slot.

6. The antenna structure defined in claim 5, further comprising meanselectrically loading the antenna including conductive rod-like membersprojecting from selected transverse members of the zigzag element towardthe back; side of the cavity but terminating shortfof ,making electricalcontact therewith, therebyto efiect artificial dielectric loading ofsaid cavity.

defined in claim .3, wherein '7. The antenna structure the zig-zagelement is formed in two partsseparated from each other substantiallymidway between the ends,

each part being electrically connected with opposing polarity to theenergy transmission means. I? 1 8. The antenna structure defined inclaim 3, and refiector means comprising a shallow conductivesurfacemounted behind theslot at less than aquarter-wavelength spacing. r

The antenna structure defined in claim 18,;f urther comprising meanselectrically loading the antenna including conductive rod-like membersprojecting from se lected transverse members of the zig-zag elementtoward the reflector means surface but terminating short of makingelectrical contact therewith, thereby to effect artificial dielectricloading of said cavity.

10. The antenna structure defined in claim 3, wherein the zig-zagelement is energized by the energy transmission means connected to oneend thereof and to an adjacent location on the conductive surfaceelement.

11. The antenna structure defined in claim 10, wherein the slottedconductive surface element is also energized by the energy transmissionmeans connected to the same at corresponding locations along thelongitudinally extending slot edges, thereby to dually polarize theantenna structure.

12. The antenna structure defined in claim 3, wherein the slottedconductive surface element is energized by the energy transmission meansconnected to the same at corresponding locations along thelongitudinally extending slot edges.

13. Electromagnetic antenna structure comprising two radiating elements,one a means presenting a conductive radiation surface having anelongated slot therein, and the other a zig-zag conductive elementmounted in said slot and presenting a broadside radiation face insubstantially coplanar relation to said surface, said zig-zag elementhaving a plurality of members extending transversely to the length ofsaid slot and successively interconnected at alternately opposite endsto define an outline configuration contained wholly within the outlineof said slot and all portions of which are marginally spaced inwardlyfrom the edge of said slot on all sides, and energy transmission meanselectrically connected to both of said elements to energize said antennafor dual polarization.

14. The antenna structure defined in claim 13, wherein the zigzagelement is formed in two parts separated from each other substantiallymidway between the ends, each part being electrically connected withopposing polarity to the energy transmission means, and wherein theslotted conductive surface element is also energized by the energytransmission means connected to the same at corresponding locationsalong the longitudinally extending slot edges, thereby to duallypolarize the antenna structure.

15. Electromagnetic antenna structure comprising two radiating elements,one a means presenting a conductive radiation surface having anelongated slot therein, and the other a relatively thin sheet-likezig-zag conductive element positioned in said slot and presenting abroadside radiation face in substantially coplanar relation to saidsurface, said zig-zag element having a plurality of members extendingtransversely to the length of said slot and successively interconnectedat alternately opposite ends to define an outline configurationcontained wholly within the outline of said slot and all portions ofwhich are marginally spaced inwardly from the edges of said slot on allsides, said slot being occupied by a body of dielectric material acrossits breadth and width supporting said zig-zag element so positionedtherein, and energy transmission means electrically connected to atleast one of said elements to energize said antenna.

16. Electromagnetic antenna structure comprising an elongated zig-zagconductive element having a'iplurality of members extending transverselyof the length of said element and successively interconnected atalternately'oppos'ite ends, the transverse width of said element beingminimum intermediate its ends and increasing exponen- -tially'to itsrespective ends, and the thickness of and spacing between thesuccessively adjacent transverse members being minimum substantiallywhere the transverse width-of the element is minimum and increasing"logarithmicallytoward opposite ends of said element, and

means electricallyconnected to said element for energizing the same.

. References Cited in the file of this patent UNITED STATES PATENTS 2,751,589

OTHER REFERENCES 'Kraus: Antennas, McGraw-Hill Book Co., NewYork,

1950, page .368.

"Du'Hamel and Isbell: Broadband Logarithmically Periodic AntennaStructures, March 1957, I.R.E. National Convention Record, ,part I. pp.119-128.

